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PHOSPHORIC ACID                                  3i

1.  Sulphates, Chlorides.—Separate portions of the i: 10 solution,
acidified with nitric acid, should not become turbid with barium chloride
or silver nitrate.

2.  Ammonia.—2 grams, heated with excess of caustic soda, should
yield no ammoniacal odour.

2 grams dissolved in 30 c.c. of water, neutralised with sodium
hydroxide and made up to 50 c.c., should give no coloration with 10-15
drops of Nessler solution.

3.  Metals.—The i: 10 solution should not become turbid (calcium)
when rendered alkaline with ammonia or after further addition of ammonium
sulphide (copper, lead, iron).

10 grams, heated in a platinum crucible, should volatilise without
turning brown or leaving appreciable residue. Any residue left should be
tested for alkali metals.


H3PO4 = 98-024 (98)

Ordinary phosphoric acid or orthophosphoric acid is a colourless, odour-
less liquid with density varying according to the concentration (1-73 =
90% ; 172 = 86% ; 1-44 = 60% ; 1-35 = 50% ; 1-26 = 40% ; 1-15 =
25%). The impurities to be tested for in the commercial acid are : sul-
phuric, nitric, hydrochloric, metaphosphoric and phosphorous acids,
ammonia, arsenic, heavy and earthy metals and organic matter.

1.  Sulphuric Acid.—The dilute acid (1:2)  is treated with barium
chloride in the hot:   no turbidity should appear, even after standing.

2.  Nitric Acid.—The dilute acid (i : i) is treated with a few drops
of a sulphuric acid solution of diphenylamine:  no blue coloration should
appear.    I   c.c.  of the acid + 3 c.c. of water should not decolorise a
drop of a sulphuric acid solution of indigo.

3.  Hydrochloric Acid.—The acid, diluted with 5 vols. of water and
treated with silver nitrate in the cold, should give no turbidity.

4.  Phosphorous Acid.—To the acid, somewhat diluted, silver nitrate
is added and the liquid heated to boiling ; in presence of phosphorous acid,
blackening is observed.    Also, the acid is heated with mercuric chloride,
which gives a white precipitate of calomel in presence of phosphorous acid,

5.  Metaphosphoric Acid.—The diluted acid is added to a dilute solu-
tion of albumin :  a turbidity is formed if metaphosphoric acid is present.

6.  Ammonia.—The acid is heated with excess of caustic soda and the
odour of the evolved vapour noted.

7.  Arsenic.—When tested for an hour in the Marsh apparatus (see
Flesh Foods, Vol. II), the acid should give no arsenic ring;   i c.c. of the
acid with 5 c.c. of Bettendorf's reagent (see note on p. 18) should give no
coloration within an hour.

8.  Heavy and Earthy Metals.—The acid is subjected to a cuirent
of hydrogen sulphide.    Another portion, greatly diluted, is rendered alkaline
with ammonia and then tested with ammonium sulphide, ammonium 2-5 of